Various types of optical test apparatus are known, which project a beam of light against a surface, and then checks the reflected or return beam. Such apparatus operates contact-less, and is used, for example, to check the surface characteristics of an object, such as a workpiece in the micrometer range. Modern types of such apparatus utilize optical systems whicn are well-known and commercially available from compact disk (CD) players. CD players utilize light pens, secured in a housing of the apparatus. An objective is located close to the facing ends of the housing, movable between two spaced leaf springs along the optical axis. The objective is focussed by an electrodynamic drive system which shifts the objective counter the spring action of the leaf springs along the optical axis until a predetermined level of focussing is obtained. Focussing is checked by the light pen by using a plurality of photo-transducers, such as photo-diodes or photo-transistors. The image of an illuminated spot, derived for example from a laser, is projected by the objective and a collimator on a surface over which the plurality of photo-diodes or photo-transistors are distributed. The light source projects a beam of light from a laser through the collimator and the objective on the surface to be tested. The image of the light spot is enlarged on the photo-diodes, with respect to the light spot projected on the test surface in order to obtain high resolution.
Changes in focus occur if the height profile of the test object changes upon focussing of the objective. Such changes in focussing are then compensated by repositioning of the objective lens. The extent of repositioning of the objective then will be a measure for the height or level dimension of the profile of the test object at tne spot where it is illuminated by the light beam from the laser.
The deflection of the objective is measured by a distance measuring apparatus which, also, is contact-less and operates inductively. An armature is provided, which moves parallel to the optical axis of the objective.
A construction well-known in the prior art provides for emmission of the test beam at one of the facing ends of the housing. This arrangement restricts use of the apparatus to comparatively large, plane surfaces. Edges or rims interfere if they have a lesser spacing from each other than the cross-sectional dimension of the housing, measured at right angles to the optical axis. The apparatus is not capable of measuring the surface configuration of openings, for example in tubes, or bore openings having a diameter which is less than the height of the housing.
Placing the drive arrangement for the objective close to the objective causes heat effects to occur which, in use of the system for CD players, is not critical; when used as a testing or measuring element, however, such heating effects may introduce measurement errors.